Chlorine and caustic are essential, large volume commodities used in all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal chlorides with the largest portion of such production coming from mercury and diaphragm cells. With the advent of technological advances such as dimensionally stable anodes, high activity catalytic cathodic materials and cation exchange, hydraulically impermeable permselective membranes, considerable improvements have been made in both product quality and energy efficiency. However, the complicated chemical structure of these membranes and their relative fragility make it difficult to optimize production parameters. For this, smaller cells are used to determine basic membrane characteristics, particularly their cationic and water transfer numbers and their dynamic properties under conditions typical of an operating cell.
Extensive literature exists on the determination of ionic and water transport numbers for ion exchange membranes. For cationic transport both Hittorf-type electrolysis experiments and indirect emf methods have been used. In similar fashion, membrane water transport numbers can be measured by electrolysis techniques or by streaming potential techniques. Aside from the systematic discrepancies which have been observed between emf and the true electrolysis results, the former techniques do not lend themselves to studies using a high current density.
Electrolysis methods based on measuring changes in either electrolytic solution volume or weight are known. Volume methods are generally more convenient, but are susceptible to error due to membrane movement and are difficult to use at elevated temperatures. With this approach, the best reported measurements of potassium ion transport numbers, at room temperature and low current density, have an average relative standard deviation of about 6%.
The need to create a measurable concentration change during electrolysis with this approach presents a further problem for cationic transport number measurements in concentrated solution environments. If concentration changes are kept small, it is difficult to obtain sufficient accuracy in solution analysis to obtain a reliable result. If larger concentration changes are produced, such membrane properties as water and electrolyte content are altered with the result that interpretation of the results becomes considerably more difficult.
It has been shown that the use of radioactive tracer techniques can be effective in removing the problem of concentration changes in the measurement of membrane transport parameters. These techniques have led to a considerable improvement in the accuracy with which measurement of membrane characteristics under conditions typical of those used in production cells can be made. A description of prior art methods and an improved test cell for performing such measurements by this technique are described in U.S. Pat. No. 4,379,029, issued to Howard L. Yeager on Apr. 5, 1983, and assigned to Olin Corporation. However, even with this cell, it is still necessary to perform all operations manually, which both limits the complexity of the experimental procedures which can be used and greatly increases the time (about one full day) required to perform a measurement on any given membrane with any given set of test conditions. Part of this problem has been solved in a system described by Peter J. Smith and Trevor L. Jones in "The Development and Application of a Small Cell for the Determination of Ionic Transport Numbers by Use of Radiotracers" presented at the American Chemical Society (Polymer Division) Workshop on Perfluorinated Ionomer Membranes held in February, 1982 at Orlando, Fla. However, the semi-automatic test apparatus described therein is unable to programmably vary experimental parameters such as the brine or caustic concentrations or current density so that many of the limitations described with the fully manual systems are still present.
Most recently it has been observed that for more complete optimization of membrane cell operations, especially at higher current levels, the power absorption properties of the membrane must also be considered. None of the devices described above provide such a capability.